The invention relates to a focus detection system, and more particularly, to such system in which an in-focus position of an optical instrument such as a camera, microscope or the like is detected by an electrical processing of a picture signal from an object being observed.
A conventional technique for detecting an in-focus position of an optical instrument such as photographic camera, microscope or the like utilizes a light acceptor assembly having a plurality of discrete light acceptor regions on which an optical image of an object being observed is projected to derive picture information from the respective regions, which information is then used as a basis to calculate the magnitude of a given evaluation function, which is in turn effective to determine whether or not the image of the object is focussed on the regions.
The evaluation function serves as a means for evaluating the sharpness of an image, and a variety of such functions are proposed. Typical functions include ##EQU1## where K represents the number of light acceptor elements, x.sub.k represents a photoelectric output from K-th element, x an average value of photoelectric outputs from the light acceptor elements, .nu. the spatial frequency of an image flux.
The evaluation function S of the expression (1) signifies that the sharpness of an image is evaluated in accordance with the total sum of absolute values of the differences between outputs from adjacent picture elements while the evaluation function S of the expression (2) signifies that the sharpness of an image is evaluated in accordance with an average of absolute values of differences between individual outputs from the respective picture elements and an average value. Finally, the evaluation function S of the expression (3) utilizes a power spectrum of spatial frequences of an image flux which is chosen as the basis to evaluate the sharpness of an image.
The light intensity distribution of an image produced in an optical instrument such as a photographic camera, microscope or the like exhibits a variety of forms. A focussing operation takes place by utilizing a limited region such as the boundary between an image of an object and its background where the distribution of the light intensity is frequently represented by a stepped distribution, as illustrated in FIG. 1. The focussing by sight also is based on the distribution of the light intensity in such region. Consequently, a function which effectively evaluates such stepped distribution of the light intensity is most desirable for practical purposes.
FIG. 2 graphically shows the distribution of the light intensity of an image when in focus where an image plane is located within the depth of focus of a focussing lens for an object exhibiting the stepped distribution as shown in FIG. 1 as well as such distribution when defocussed where the image plane is located out of the depth of focus. The curve A shown in solid line represents such distribution when in focus while the dotted line curve B represents the distribution when defocussed.
If the function of the expression (1) is used as an evaluation function for an image exhibiting such stepped distribution, the resulting curve which plots the magnitude of the evaluation function over varying positions will be represented by curve C shown in dotted lines in FIG. 3. The curve C is approximately rectilinear over a wide range including the in-focus position, so that no significant difference is recognized between an in-focus condition and a defocussed condition. Hence, it is apparent that such function is not suitable to evaluate the image which exhibits a stepped distribution of light intensity. When the evaluation function of the expression (2) is employed, the resulting curve will be gently curved as indicated by phantom line D in FIG. 3. Though a difference is recognized between an in-focus condition and a defocussed condition, the magnitude of the difference is insignificant. In particular, in a region adjacent to the in-focus position, the curve D approaches a rectilinear trace, so that an accurate focussing operation is difficult to achieve. The use of the evaluation function of the expression (3) provides a solid line curve E shown in FIG. 3 which exhibits a steeply rising peak toward the in-focus position, enabling a clear distinction between the in-focus and defocussed conditions. However, the involved calculation is very complex, which is a great disadvantage.